Rogil22,
Your questions are all interdependent so they can't be answered one by one.
I guess it all boils down to what H2S concentration is the typical limit above which you'd experience pyrophoricity issues when opening equipment.
The concentration is actually quite low. In my experience, you may already have pyrophoricity problems in a hydrofiner reactor treating feed with ~1000 ppm sulfur when you pull the scale traps.
The heat up rate is dependent on the dispersion (surface exposed to oxygen) of the iron sulfide. Scale traps catch fire quickly, but I've also seen a heat exchanger heat up slowly overnight (!!) to reach a temperature at which it became literally red-hot and the tubes were all bent.
That means that the most dangerous scenario is actually the one where nothing seems to happen first and equipement is left somewhere in the unit for several hours while nobody is paying attention.
The objective should be to identify all units with potential pyrophoricity problems and regularly monitor the status of any equipment opened or pulled out.

At the Laurance Reid Gas Conditioning Conference this year in Oklahoma, there was a very good presentation on iron sulfides put on by Ben Spooner of Amine Experts.  If you contact him though their website, he may be able to provide some help.

www.amineexperts.com

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals
www.quadrachemicals.com

“Pyrophoric Iron Sulphides
Pyrophoric iron sulphides form when iron is exposed to hydrogen sulphide, or any other compound that contains sulphur, in an oxygen deficient atmosphere. They are found frequently in vessels, storage tanks, and sour gas pipelines. Pyrophoric iron sulphides present a hazard when equipment and tanks are opened for cleaning, inspection, and maintenance. As the iron-sulphide compounds dry out and come in contact with air, they react with the oxygen and spontaneously ignite.
The reactivity of an iron sulphide depends on the type of iron oxide from which it was derived. Reactive iron sulphides can be deactivated when wetted with oil, therefore, rusted surfaces that are below the oil level are at low risk of causing an explosion.
Chemical and mechanical methods are available to remove iron sulphides. The use of potassium permanganate is gaining acceptance for this purpose because it improves safety, saves significant cost and increases productivity. Other treatments include acid washing, chemical suppression, and the use of high-pH reagents.

General Precautions to Avoid Pyrophoric Iron Fires
1.    The scraps and debris collected from cleaning of filters in naphtha / crude service must be kept wet and disposed of underground.
2.    Tanks, reactors, columns, and exchangers in high-sulfur feed service must be kept properly blanketed with N2 during idle periods.
3.    All equipment and structured packing must be properly water washed and kept wet when exposed to the atmosphere.
4.    In processes where catalyst handling is required (such as in Hydrotreating and fluid catalytic cracking) caution must be taken during catalyst recharge or disposal. When unloading any spent coked catalyst, the possibility exists for iron sulfide fires. If the spent catalyst is warm and contacts oxygen, iron sulfide will ignite spontaneously and the ensuing reaction may generate enough heat to ignite carbon deposited on the catalyst. Therefore catalyst must be stripped of all hydrocarbons, cooled to about 50 o C and wetted with water to prevent it from igniting vapors. Once cooled, the used catalyst may be emptied into drums for later shipment to a regenerator or a disposal site. As the catalyst may be highly pyrophoric (containing iron sulfide, etc.), it should be dumped into drums containing an internal liner for shipment. The drum and liner should first be filled with inert gas, which is then displaced by the catalyst. The liner should be tied off and a small chunk of dry ice placed inside the drum before sealing. These precautions should protect against catalyst auto ignition.”

Flare lines downstream of vessels with sulfurous contents should invariably be suspected of building pyrophoric iron over time.

I worked at an old Shell refinery in the 1960s where the flare line had been opened for maintenance.  Several staff were standing directly downstream of the line and suddenly there was a huge ball of flame that shot out of the flare line and caused several extremely severe, disfiguring burns (including, ironically, the side of the body and face of the fire and safety department chief).  The cause of the flame was was determined to be pyrophoric iron, burning spontaneously on contact with air, which set off some hydrocarbon liquids still in the line.